KNIT LINE FRACTURES IN INJECTION MOLDED LIQUID-CRYSTALLINE POLYMERS

The microstructure and fracture behavior of injection molded samples of unfilled and filled grades of liquid crystalline polymers (Vectra(R) and Ultrax(R)) containing cold and warm knit lines have been studied by scanning electron microscopy, X-ray scattering, and Instron tensile tests. Four process parameters-melt temperature, mold temperature, injection time, and holding pressure-have been varied in accordance with a 2(4) factorial design experiment. In cold knit lines a 200-mu-m thick region with transverse orientation with Hermans orientation function f almost-equal-to 0.12-0.18 is formed on both sides on the weld area. The strength of the cold knit lines is very low, 9.0-19.8 MPa corresponding to 15-20% of the full strength of the material. The strength of the warm knit lines increases markedly with increasing distance from the insert and is generally significantly greater (40-45 MPa, 48 mm from the insert) than in the cold welds. Annealing at 260-300-degrees-C of samples containing cold knit lines causes first a partial healing of the knit line and later extensive chemical degradation of the polymer.